PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 8rv825dykpx1m, child number 0
-------------------------------------
select a.*,(select name from b where b.id=a.id) from a
Plan hash value: 2657529235
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| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------
|* 1 | TABLE ACCESS FULL| B | 9 | 1 | 9 |00:00:00.01 | 63 |
| 2 | TABLE ACCESS FULL| A | 1 | 2 | 9 |00:00:00.01 | 8 |
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Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("B"."ID"=:B1)
Note
-----
- dynamic sampling used for this statement
22 rows selected.
b.name字段全部为‘b1’，由上面的执行计划可以知道，b表执行9次，返回9行
SQL> update a set id=1;
9 rows updated.
SQL> commit;
Commit complete.
SQL> select * from a;
ID NAME
---------- --------------------
1 a1
1 a2
1 a3
1 a4
1 a5
1 a6
1 a7
1 a8
1 a9
9 rows selected.
SQL> select * from b;
ID NAME
---------- --------------------
1 b1
2 b1
3 b1
4 b1
5 b1
7 b1
8 b1
9 b1
9 rows selected.
SQL> select a.*,(select name from b where b.id=a.id) from a;
ID NAME (SELECTNAMEFROMBWHER
---------- -------------------- --------------------
1 a1 b1
1 a2 b1
1 a3 b1
1 a4 b1
1 a5 b1
1 a6 b1
1 a7 b1
1 a8 b1
1 a9 b1
9 rows selected.
SQL> @getplanspe
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 8rv825dykpx1m, child number 0
-------------------------------------
select a.*,(select name from b where b.id=a.id) from a
Plan hash value: 2657529235
------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------
|* 1 | TABLE ACCESS FULL| B | 1 | 1 | 1 |00:00:00.01 | 7 |
| 2 | TABLE ACCESS FULL| A | 1 | 2 | 9 |00:00:00.01 | 8 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("B"."ID"=:B1)
Note
-----
- dynamic sampling used for this statement
22 rows selected.
SQL>
关联字段a.id全部为1，a表有9行，标量子查询相当于执行9次select name from b where b.id=1 ，oracle也不傻，starts=1，说明只执行了1次。
总结：
理想状态下，a.id为主键，没有重复值，那么a表返回多少行，b表就要被执行多少次。
特殊情况下，a.id的distinct值只有n个，那么b表只执行n次。